Wide band amplifier



April 16, l9 46. w. E. BRADLEY WIDE BAND AMPLIFIER Filed April 28,; 1944 TERN/NAT/N INVENTOR ATTORNEYS ,pinned All. is, 194e FICE wmE BAND AMPLIFIER y William E. Bradley, Swarthmore, Pa., assignor to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation oi Delaware Applica/tion April 28, 1944, Serial No. 533,225

6 Claims.

This invention relates to ampliiiers, and more particularly to wide-band amplifiers which are constructed and arranged to introduce no time delay at any frequency within a predetermined transmission band.

One of the recognized effects produced by the conventional amplifier stage is that the phase of the amplifier-s output signal is shifted with respect to the phase of the input signal. The phase shift produced by a given amplifier is, in generalfa function of the signal frequency. This phase, shift increases with the number of stages. In many applications phase shift is not disadvantageous so long as the phase shift produced by the amplifier increases linearly with frequency. This condition is dlillcult to obtain, however, partleula'rly in wide-band multi-stage amplifiers, and consequently eiorts in this direction have not always'been satisfactory. In other instances, the

presence of any phase shift whatever, Whether linear or not', is disadvantageous. The present invention is directed toward novel means for preventing theoccurrence of time delay or phase shifts in ampliiiers.

Accordingly it is a. principal object of the present invention to provide an amplifier which is capable of transmitting a predetermined band of signal frequencies 'without substantial time delay.

Itl is another object of the present invention to provide an improved coupling impedance for use between vacuum tube stages, which impedance is constructed and arranged to present a pure resistance at all frequencies within a predetermined transmission band.

It is a further object' of the invention eiectively to overcome the deleterious effects of the plate-output and grid-input capacities of vacuum tubes.

.It is another object of the invention to proa vide a novel coupling impedance, which impedance is capable of absorbing certain undesired y shunt capacitances inherently associated with the circuits to be coupled.

It is still another object of the invention to provide a novel load network for use with vacuum tubes, which network shall have a substantially constant resistive impedance throughout a desired or predetermined transmission band.

Other objects and purposes of 'the invention will. be apparent to those skilled in the art from the following description and the accompanying drawing,in which: c

Fig. l is a schematic wiring diagram of a pair.

of vacuum tube amplifier stages coupled by means of an impedance Z;

Fig. 2 is a block diagram of an improved form of the impedance Z of Fig. l;

Fig. 3 is a schematic diagram of the load impedance of the present invention in a basic and highly developed form;

Fig. 4 is an explanatory diagram which represents graphically certain of the `impedance-versus-frequency characteristics of elements of the present invention:

Fig. 5 isa schematic diagram of a more practical form of the invention Fig. 6 is a schematic circuit diagram ofy a simpliiied coupling network; and

. Fig. 7 is a. schematic circuit diagram of a highly simplified coupling circuit.

Reference is iirst made to the circuit of Fig. 1 in which the environment of the invention is illustrated. This circuit comprises a pair of vacuum tubes V1 land Vn which are coupled, in conventional manner, by means of the plate load irnpedance Z and the coupling condenser C2. Plate 'current may be supplied to the ampliiier tube V1 by means of a suitable battery or other source (not shown) connected to the terminals designated B- and B+. The present invention relates to an improved impedance Z, the terminals of which are designated :c a: in Fig. 1. In the interest of simplicity the vacuum tube amplifier circuit will not be repeated in the subsequent ngures, but the terminals of the improved coupling network, which forms the basis of the present application, will also be designated .r and it will be understood that such coupling networks may be connected in the circuit of Eig. 1 in place of the generalized coupling network Z.

Fig. 2 shows, diagrammatically, an improved plate load or coupling network constructed in-accordance with the principles of the present in vention. According to the invention the improved load impedance comprises the parallel combination of the midseries input impedance, Zz, of a lter terminated in its characteristic im# pedance, and the mid-shunt input impedance, Z2, of a second filter terminated in its characteristic impedance. Preferably both iilters should be constructed and arranged to have like characteristic impedances, and like cut-off frequencies.

0 As will be shown hereinafter the parallel combination of4 such a pair of impedances Z1 and Zz, may be a nearly constant pure resistance throughout a predetermined transmission band.

The basic schematic form, or prototype, ci the improved impedance illustrated diagrammatically in Fig. 2is be understood that more complex types, such as the m-derived types, may also be used to advantage. In Fig. 3 the portion of the system to the left of the terminals :c-- represents, schematically, the impedance Z1, while the portion of the circuit to the right of the terminals :crepresents, schematically, the impedance Zz.' The impedance Z1 consists, it will be observed, of a conventional low pass filter having a conventional mid-series input section. That is, the input sec- 'tionV includes a series inductance element whose inductance is half that of the other series inductance elements. The filter may be of the constant k type and there may be as many sections, each comprising series inductance L and shunt capacity C, as may be desired. The filter, if it is not infinitely long, may be terminated conventionally, in its characteristic impedance, through the agency of a conventional terminating section 2.

The portion of the load impedance to the right of the terminals :n-z comprises a conventional low-pass lter section having a conventional mid-shunt input section. 'That is, the input section includes a shunt capacitance ofy half the capacity value of the other shunt capacitance elements. This filter may also be of the constant k type, and there may be as many sections comprising series inductance L and shunt capacity C as maybe desired. This lter may also be terminated conventionally, through the agency, for example, of any suitable terminating section 4.

It is emphasized that the individual lters utilized in the embodiments of Figs. 2 and 3 are, in themselves, entirely conventional, and consequently it is deemed unnecessary to repeat here any of the highly developed and well-known lter theory which will be used by those skilled in the art in the practice of the present invention. The lters'employed may, of course, consist of any desired number of sections, each having a,

nominal characteristic impedance Z0 equal to the square root of L/C. Both the mid-series input lter Z1 and the mid-shunt input lter Zz should be designed to have the same nominal characteristic impedance, Zn, and both should have the same cut-oi frequency, fc. As used in the specification and in the claims, the term Zo, or "nominal characteristic impedance, refers to the input impedance of a properly terminated filterI network at or near zero frequency (see Fig. 4)

The basic concepts of the invention, as ernbodied in Figs. 2 and 3, are best described with reference to the explanatory diagram of Fig. 4. The impedance characteristic 5--6 shows how the mid-series image impedance of a filter having a mid-series input section varies with frequency between the limits of zero frequency (direct current) and a frequency well above the cut-0K frequency, fc. of the filter. Within the range f=0 to fc the impedance of such a filter is purely resistive, being equal to Zo at zero frequency, falling off gradually to zero at the cutoif frequency, fe, at which point the series input inductance resonates with the'iirst shunt capacitance. Beyond this frequency the filter exhibits an inductive reactance, the impedance rising generally as shown.

The impedance characteristic'- shows how the mid-shunt 'image impedance of a filter having a mid-shunt input section varies with frequency. Within the range from f=0 to the cut-olf frequency, fe, the impedance of such a filter is also purely resistive, and rises from the characteristic impedance Zo, approaching infinity illustrated in .detail vin Fig. a. It wur at fe. Beyond fs the lter exhibits a capacitive reactance, and the impedance of the filter drops ofi as shown, approaching zero impedance asymptotically. 1

By the present invention an improved plate load, or coupling, impedance is provided by connecting in parallel the input terminals of a pair of lters having characteristics of the type illustrated at 8 6 and 8-8 in Fig. 4. The parallel combination of two Isuch filters results in a load network having a zero-frequency impedance of one-half Zo. Since the impedance of the two filters individually is purely resistive below the cut-o frequency, it follows that the impedance of the two filters in parallel is also purely resistive in this range. Moreover, `since the individual input impedances of the two filters deviate in opposite senses from` their zero-frequency impedanceas fc is approached, it also follows that the input impedance of the two filters in parallel will be much more nearly constant throughout the pass-band. This desirable effect is clearly shown in Fig. 4 by the combined impedance characteristic l-lll-IIL Beyond the cut-off frequency fc, the impedance of the network becomes reactive, approaching infinity at the frequency where the impedance characteristics 6 and 8 intersect. In the drawing this frequency is designated fr. At the latter frequency the impedance of the network may be so high as to produce singing of the amplier system, and consequently it is usually desirable to limit this impedance and thus to forestall such tendency by damping the network. This may be readily accomplished by shunting a suitable damping resistor l2 across the input terminals :v -:c of the network. In addition it may be desirable tol insert a series damping resistor I4 in the connection to the inductance element L/2 of the mid-series input section of the lter extending to the left of the terminals :v -:v Preferably the resistance of the shunt damping resistor I2 should be high compared to the qsuare root of L/C. The resistance of the series damping resistor i4 should, on the other hand, be low compared to the square root of L/C. It will be evident that, if desired, the shunt damping eect may be taken care of by the grid leak resistor i8, Fig. l, in which case the resistor i2 may be omitted.

The load impedance Z presented by the network of Fig. 3 is evidently a substantially constant resistance over the frequency band extending from zero frequency (direct current) to some upper limit just below the cut-ofi.' frequency fc of the individual filters. Accordingly a signal voltage, whose frequency lies within this range, will experience no time delay in passing through an amplier employing plate load impedances of this type.

. Returning to Fig. l, the load impedance Z is, in practice, inevitably shunted by a capacitance which is due to the.distributed capacitance to ground of the leads and of the associated tube elements. This deleterious shunt capacity is represented in Fig. i by the dashed-line condenser I6. According to the present invention this deleterious capacity may be effectively absorbed by simply reducing the value of the input condenser C/2 of Fig. 3 by the capacity of the dele' terious capacity I6. If, perchance, the capacity i6 should just equal the capacity C/2 of Fig. 3, the latter may be omitted entirely.

It will be understood, of course, that where D. C., or very slowly varying, voltages are to be transmitted by the amplifier of Fig. 1, the coupling condenser Ca must be removed, and resort had to a conventional conductive coupling arrangement.

In general it is possibley to approach the'ideal form of the invention to a sucient degree of ac-` curacy by means oi' networks which are much simpler than that illustrated in Fig. 3. Such a simpli- -fled network is illustrated in Fig. 5. The lter netsection. 'I'he terminations oi' both filters include the customary shunt resistor R whose resistance is equal to the square root of L/C. It may be observed that in the circuits of Fig. 5 (et seq.) the mid-series input lter and the mid-shunt input iilter are terminated differently, Thus in Fig. 5 the former is provided with a conventional midseries terminating section, while the latter is provided with a conventional mid-shunt terminating section. It will beevident to those skilled in the art, however, that both nlters may be similarly terminated, it being necessary only to adhere to the usual filter design conventions.

The plate load impedance illustrated schematically in Fig. 6 represents a further simplification of the system of Fig. 3. In Fig. 6, as well as in certain of the other ilgures, not all of the reactance elements have been specically dimensioned. In general only certain key dimensions which will serve to identify the system to those skilled in the art have been set forth in the drawing. It will, of course, be understood that the invention is not limited to the specific values designated in the drawing.

The embodiment of Fig. 7 represents a still further simplification. While this modification will give an impedance which is fairly constant and approximately resistive over the entire passband, its performance is less desirable than that of the previously described embodiments.

It will be evident that the invention is not limited to the specic forms and embodiments illustrated, since further -modiiicatlons falling within the spirit and scope of the present invention will be apparent to those skilled in the art.

I claim:

1. In a vacuum tube amplifier, a pair of vacuum tubes, a two-terminal coupling impedance connected as a load impedancein the output circuit of the ilrst of said tubes. and means coupling the input electrodes oi' the other of said tubes to the two terminals of said load impedance, said load impedance comprising the parallel combination of a first filter network provided with a mid-shunt input section and a second illter network provided with a mid-series input section.

2. Ina wide-band vacuum tube ampliiler, a pair of vacuum tubes, a two-terminal coupling impedance connected as a load impedance in the plate circuit of the rst of said tubes, and means connecting the input circuit of the otherof said tubes in shunt with said load impedance, said load impedance comprising a mid-series input lter connected in parallel with a mid-shunt input illter, whereby said coupling impedance presents a purely resistive, substantially constant impedance overV a predetermined wide range oi' frequencies.

3. In a. vacuum tube amplifier, a pair of vacuum tubes, a two-terminal coupling impedance connected as a load impedance in the plate circuit oi' the rst' of said tubes, and means coupling the input electrodes of the other of said tubes to the two terminals of said load impedance, said load impedance comprising a pair of terminated lowipass lters having their input circuits connected in parallel, said parallel-connected input circuits providing the two terminals of said coupling impedance, the first reactive element of one of said filters being a series inductive element, the rst reactive element of the other oi' said lters being a shunt capacitive element, the Vcapacitance of said capacitive element being less than the normal computed value by substantially the magnitude of the deleterious capacitances in shunt with the associated tube elements. I u

4. In a vacuum tube amplifier, a pair of vacuum tubes, va two-terminalcoupling impedance connected as a load impedance in the plate circuit of the rst ofsaid tubes, and means connecting the input electrodes of the other of said tubes to the two terminals of said load impedance, said load impedance comprising the parallel combination of the mid-series input impedance of a ilrst lter terminated in its characteristic4 impedance, and the mid-shunt input impedance of a second illter terminated in its characteristic impedance.

5. A vacuum tube amplifier as claimed in claim 4. characterized in the application of dissipative means to at least one of said illters to prevent "singing" at frequencies outside the pass-band of said illters.

6. A vacuum tube amplier as claimed in claim 4, wherein both filters have substantially the same nominal characteristic impedances. and both have substantially identical pass-bands.

`WILLIAM E. BRADLEY. 

